The invention relates to a multibeam antenna, and in particular to a transmit and/or receive multibeam antenna for satellite applications, designed to operate in the microwave part of the spectrum (300 MHz-300 GHz).
It is well known in the art of antenna engineering that the generation of directive beams implies using antennas with large electric dimensions, usually based on reflectors.
A conventional solution for generating a coverage characterized by contiguous high directivity spot beams consists in using several reflector antennas—typically three or four in reflection and the same number in transmission—in order to generate interleaved beams. See S. K. Rao “Parametric Design and Analysis of Multiple-Beam Reflector Antennas for Satellite Communications”, IEEE Antennas and Propagation Magazine, Vol. 45, No. 4, August 2003. This type of architecture presents severe problems of accommodation when used onboard satellites.
Phased arrays may allow generating a multibeam coverage using a single aperture. However they are very expensive, due to the high number of radiating feeds constituting the array and to the need for a complex beam-forming network.
Another possibility consists in adopting an antenna system based on microwave lenses. According to this approach, each beam is generated by a single feed, which is disposed on the focal surface of a lens; the field generated by each feed is converted by the lens into a directive beam. Conventional dielectric lenses are too heavy and lossy for large aperture antennas, and they require at least one curved surface, which make them difficult to manufacture. Moreover, large dielectric elements should be preferably avoided in satellites.
Discrete or “constrained” lens antennas constitute an interesting alternative to dielectric lenses.
A “discrete” or “constrained” or “bootlace” lens concept is illustrated in the paper by D. McGrath “Planar Three-Dimensional Constrained Lenses”, IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 1, January 1986; see also document U.S. Pat. No. 3,984,840.
A discrete lens is basically constituted by a first array of radiating elements (“back array”) and a second array (“front array”) comprising the same number of radiating elements. Each element of the front array is connected to a single element of the back array via a respective waveguide or transmission line connection. This way a microwave signal received by an element of the back array propagates to the front array and is reemitted by the corresponding element of the front array (in the case of a transmitting antenna; the reciprocal is true for an emitting antenna). The connections have different lengths and therefore introduce different phase shifts. If the length of the connections going from the center towards the edges of the arrays is properly designed and if a particular relationship between the positions of corresponding radiating elements in the front and back array is satisfied, then the whole structure behaves like a converging lens.
Feeds (e.g. horn antennas) are disposed on the focal surface of the lens, facing the back array. The ensemble can constitute either a transmit or a receive, or a transmit/receive antenna.
A drawback of passive lens antennas of this kind is associated to the significant losses introduced: indeed, a large part of the power impinging on the back array (for a transmit antenna) or on the front array (for a receive antenna) is not intercepted by the radiating elements of said array. In reception, this reduces the achievable signal-to-noise ratio of the received signal, and in transmission this leads to an unacceptable waste of electrical power. Besides, exactly like for reflector antennas, a part of the power is not intercepted by the lens aperture: the corresponding losses are known as “spillover” losses.
These problems can be solved, or at least alleviated, by introducing active elements within the connections between the front and back radiating elements of the discrete lens (i.e. low-noise amplifiers for a receive lens, power amplifiers for a transmit lens). This way, the lens antenna becomes an Active Lens Antenna. This solutions is disclosed by the paper by S. Hollung and Z. B. Popovic “A bi-directional active lens antenna array”, Antennas and Propagation Society International Symposium, 1997 IEEE, 1997 Digest Volume 1, 13-18 Jul. 1997 Page(s): 26-29, vol. 1.
While active lens antennas are simpler than phased array antennas because they do not require a beam forming network, they lack the flexibility of the latter. Moreover, they are still quite complex and heavy because a large number of radiating elements is required both in the front and in the back arrays.